Tetrode ion pump



' A July 1, 1969 L. T. LAMONT, JR

TETRODE ION PUMP Filed Sept. 19, 1967 FIG.|

FIG.2

NUMBER OF IONS FIG"4 PER UNIT AREA NUMBER OF To ATOMS FROM NUMBER OFOPPOSITE ATOMS v CATHOOE PER RESPUTTERED UNIT AREA PER UNIT AREAL'FOURTH ELECTRODE INVENTOR. LAWRENCE T. LAMONT, JR. BY

ATTORNEY,

United States Patent US. Cl. 230-69 8 Claims ABSTRACT OF THE DISCLOSUREThere is disclosed a four electrode getter-ion pump which provides a toincrease in argon pumping speed which speed is apparently stable overthe entire pressure range. The pump is essentially a one voltage triodewith a fourth apertured electrode positioned between the sputter elementand the anode assembly and to which is applied a negative potential ofabout -2 kv. The anode assembly and pump housing are maintained atground potential. The sputter member is maintained at a high negativepotential, as of 4 kv. The sputter member or cathode is also aperturedbut includes vanes vertically disposed such that ions which strike thesputter mem her do so at a very small angle of incidence. Those ionswhich fail to strike the vanes are buried in the surface around theaperture of the fourth electrode on a subsequent pass through the vanes.A net flux of sputtered material is shown to arrive at the area aroundthe aperture of the fourth element, such that a greater number of argonatoms are permanently pumped.

Background of the invention The present invention relates in general tocold cathode ion pumps and in particular, to a novel tetrode ion pumpfor improved pumping of noble gases.

Heretofore, vacuum pumps having an anode and a cathode have had fortheir principle of operation the establishment of a magneticallyconfined electrical discharge in and around the space between the anodeand the cathode and a strong externally applied magnetic field threadedlongitudinally through the anode. Free electrons, accelerated toward theanode in a tortuous path as a result of the magnetic field, ionize thegas to be evacuated forming positive ions of both chemically active andinert or noble gas. Both types of ions are then accelerated toward thecathode where upon impact they cause removal or sputtering of sputtercathode material. A sizeable portion of the active ions combine with thesputtered material and are permanently pumped. The inert or noble ions,however, do not form compounds but are merely buried in the surface ofthe electrode they strike. Subsequent sputtering of the surface removesthe material covering the noble ions resulting, as a consequence, intheir re-emission. Re-emission also occurs when the surface in which thenoble ions impact becomes saturated with noble ions. This is believed toresult from the fact that noble ions, though buried, are loosely bound.When the rate or re-emission of noble gas atoms becomes equal to therate of arrival of noble gas ions, the net pumping speed approacheszero.

Various proposals have been submitted for reducing the re-emission ofnoble ions resulting from sputtering and saturation. In one, anapertured electrode is placed between the anode and cathode and hasimpressed on it a voltage intermediate the voltage potential of theanode and cathode. Ions which fail to pass through the aperturedintermediate electrode are buried in the surface of the intermediateelectrode and material sputtered by other ions which pass through theaperture is deposited over the buried ions.

While satisfactory in principle, the permanent pumping of noble gasatoms in such pumps is limited to only one of the two available surfacesof the intermediate electrode, i.e., the opposing or inner surfacesnearest the anode. Further, the sputter cathode is subjected to high iondensities at a point along the axis of the anode resulting in severelocalized deterioration of the cathode requiring the addition of afourth electrode for defocusing or overfocussing the ion strains.Furthermore, the potential on the intermediate electrode shouldapproximate the free space potential otherwise the discharge intensitymay be adversely affected resulting in a reduction of ion density.

Another proposal suggests an irregular cathode surface, such as byprojections raised from the cathode, for providing increased sputteringand reduction of spontaneous periodic pressure fluctuations, however,the latter effect, often called argon instability remains undesirablysignificant.

Summary of the invention The present invention avoids the disadvantagesbriefly described above with respect to prior known vacuum pumps and asmore fullydescribed hereinafter, represents a considerable improvementin argon pumping speed and stability.

The tetrode pump to be described is essentially a one voltage triodewith an apertured fourth element positioned midway between an aperturedsputter element (cathode) and an anode assembly. The potential which isapplied to the fourth electrode is approximately one half the cathodepotential. Ions formed Within the discharge are energetically notcapable of reaching either the anode or the walls of the pump which aremaintained at ground potential and must, as a consequence, strike eitherthe cathode or the fourth electrode. Some of the ions directly strikethe inner surface of the apertured fourth electrode on the side nearestthe anode, while others pass through the aperture and are buried in theouter surface nearest the cathode. A sizeable portion of those whichpass through the aperture also pass through the cathode and strike thecathode on a subsequent pass. The ensuing sputtering produces a net fluxof sputtered material which covers the ions buried in the inner andouter surfaces of the fourth electrode, resulting in their beingpermanently pumped.

It is also proposed to use a cathode and fourth electrode preferablyconstructed of tantalum which displays special sputteringcharacteristics in that the rate of sputtering increases with decreasingangle of incidence which the impinging ions make with the cathodesurface on im pact. It has been observed that ions impacting normal to atantalum surface display a much lower sputtering yield than those whichimpact at a small angle of incidence. As a consquence, ions which impacton the surface of the cathode after passing through the cathode tend tosputter greater amounts of material than do those ions which impact onthe outer surface of the fourth electrode resulting in a net flux ofsputtered material arriving at the fourth electrode.

Since fresh material being laid over the fourth electrode surface isresputtered at a lower rate, ion re-emission due to saturation andsputtering is greatly reduced, if not entirely eliminated. Thus, argonpumping speed due to increased ion burial is improved and is apparentlystable over the entire pressure range. Referenced to the speed of air ofthe equivalent diode structure, triode pumps using the cathode structureof the instant invention have argon speeds of 20% to 25% where as thetetrode pump of the instant invention displays argon speeds of from 31%to 36% and as indicated, the speed is apparently stable over the entirepressure range.

Accordingly, a primary object of the present invention is an ion pumphaving an improved argon pumping speed which is stable over the entirepressure range.

Another object of the present invention is a tetrode ion pump in which asubstantially non-sputtering fourth electrode is positioned between theanode structure and the sputtering element.

Another object of the present invention is a pump as above describedwherein both the substantially non-sputtering fourth electrode and thesputtering element are transparent to ions.

Another object of the present invention is a pump as above describedwherein the sputtering element is made of tantalum.

Other objects, features and advantages of the invention will becomeapparent in the detailed description when taken in connection with theaccompanying drawings in which:

FIG. 1 is a diagrammatic sectional view of an ion pump embodying thepresent invention;

FIG. 2 is an exploded perspective view delineated by the line 22 of FIG.1;

FIG. 3 is a fragmentary cross-sectional view taken along lines 33 ofFIG. 1; and

FIG. 4 is a diagram of the ion and sputtered atom flux distribution inthe pump of the instant invention.

Detailed description Referring to FIG, 1, there is showndiagrammatically two cells of a novel multiple cell ion pumpincorporating the present invention which is especially suitable forpumping ditficult to pump noble gases, such as argon.

A stainless steel vacuum tight pump housing 1 provided with a gas port 2encloses a pump assembly 3. An externally applied strong magnetic fieldH is formed by magnets 7 and is oriented to thread through pump assembly3 along the longitudinal axis of a plurality of anodes 4 and throughfourth electrodes 5, 5 and cathodes 6, 6'. Preferably, the anode is madeof stainless steel, and the fourth electrodes and cathode are preferablymade of tantalum. Gas port 2 may be coupled in a vacuum tight manner toa device or chamber (not shown) desired to be evacuated. As is wellknown in the art, ionization of the gas which enters through gas port 2is caused by collision of electrons with gas atoms or molecules. Theelectrons are accelerated in tortuous paths within the anode cells underthe influence of the magnetic and electric fields. As shown in FIG. 1the anode is maintained at zero or ground potential. The aperturedsputter cathode structures 6, 6 are maintained at a high negativepotential, as of 4 kv. The apertured fourth electrodes 5, 5' aremaintained at an intermediate negative potential, as of 2 kv. The anode4 and housing 1 can be electrically interconnected but the fourthelectrodes 5, 5' and the cathodes 6, 6' are insulated from the anode andthe housing and from each other. The leads for the fourth electrodes andcathodes pass through conventional insulators 8. The positive ions areaccelerated toward the apertured fourth electrodes 5, 5 and theapertured sputter cathode structures 6, 6'.

It should be understood that the negative potential imposed on aperturedfourth electrodes 5, 5 is due to the aperture size and is independent ofthe potential on and geometry of the anodes 4 and cathode structures 6,6'. The larger the aperture in fourth electrodes 5, 5' the less effectthe potential on fourth electrodes 5, 5 has on the discharge intensity,As a consequence, the potential imposed on fourth electrodes 5, 5 may beadjusted for maximum argon pumping speed. The walls of pump housing 1are maintained at ground potential thus forming a ground plane about theother pump elements.

Referring to FIGS. l-3, it will be seen that the diameter of theaperture of the fourth electrodes 5, 5 is only about 75% of the diameterof the cells of anode 4.

Referring to FIG. 2, there is shown in greater detail a portion ofcathode structure 6. An apertured plate 10 is fixed, as by spot welding,to a vane assembly 11. A plurality of vanes 12 are held in positionnormal to the aperture of plate 10 by a slotted bracket 13. The diameterof the apertures in plate 10 approximates the diameter of the cells ofanodes 4. The vanes 12 are spaced apart such that at least three vanestraverse the aperture in plate 10. It should be understood that thespacing between the vanes 12 may be made smaller if desired such thatmore vanes traverse the aperture in plate 10. In general, the spacebetween the vanes 12 as well as their height should be large relative totheir thickness. It should be noted, however, that apertured plate 10 isnot essential to the operation of the pump so long as the vanes are ofthe dimensions and spacing indicated.

In operation, ions formed within the discharge are energetically notcapable of reaching either the anodes 4 or the walls of the pump housing1 and must, as a consequence, strike either the sputter cathodestructure '6, 6' or the fourth electrode 5, 5. Since a large fraction ofthe ions is transmitted through the apertures and vanes 12 of cathodestructure 6, 6' and strike and sputter vanes 12 on the return pass,material sputtered from said cathode structure 6 may be deposited onelements, such as auxiliary cathode 5 on the opposite end of anode 4 aswell as on the underside of auxiliary cathode 5 nearest the sputteringcathode structure 6, Of those ions which do not strike sputter vanes 12of cathode 6, some will strike the anode side of fourth electrode 5, andothers will strike the cathode side of fourth electrode 5 after passingdown and back up through vanes 12 without striking the vanes. As aresult ions will be buried in both the upper and lower surfaces offourth electrode 5. Ions buried on the lower surface of fourth electrode5 are covered by material sputtered up from cathode 6, and ions buriedon the upper surface of fourth electrode 5 are covered by materialsputtered down from cathode 6'. The described burial and covering actionoccurs in similar manner for the fourth electrode 5'. Since a net fluxof sputtered material arrives from the sputter cathode structures 6, 6'at the area of the fourth electrodes 5, 5 wherein ions are buried, theseions will be permanently pumped.

Referring to FIG. 1 and FIG. 4, there is shown a flux diagram of theions and sputtered tantalum atoms which illustrates the arrival of a netflux of sputtered tantalum atoms arriving in the area near the peripheryof each aperture in the fourth electrode 5. The basis for theillustrated flux distribution is twofold. First, the fourth electrodes 5and 5 are held at a potential less than that of the sputter cathodestructures 6 and 6'. Since, over the range of interest, the sputteringrate is a monotonically increasing function of the kinetic energy of theincident ion, the -2 kv. potential on the fourth electrodes 5, 5'results in less sputtering from these electrodes than from the 4 kv.cathodes 6, 6. Second, the sputtering rate is proportional to the ionflux at the surface and the ion flux is found to be a monotonicallydecreasing function outwardly along the radius from the axis of eachanode cell and is substantially azimuthally symmetric.

Thus, as shown in FIG. 4, the number of atoms sputtered from the fourthelectrode per unit area decreases sharply with increasing distance fromthe axis of each of the anode cells whereas the sputtered atoms arrivingfrom the opposite cathode decrease only slightly. The re sult is thatthe number of atoms of sputtered material arriving at the fourthelectrode from the opposite sputter cathode is greater than the numberleaving. A similar net buildup of sputter material occurs on the cathodeside of the fourth electrode since the rate of sputtering due to ionsimpacting normal to the surface is less than the rate of materialarriving from ions striking the sputter cathodes 6, 6' at a small angleof incidence. Radioactive xenon tracer techniques in conventional diodepumps indicate that maximum argon pumping is permanent in an area theinner radius of which is about the anode radius. Accordingly, the radiusof the apertures of the fourth electrodes 5, 5' is about 75% of theradius of the anodes 4.

Argon .pumping is further improved by constructing the sputter cathodestructure 6, 6' out of tantalum which exhibits unique sputtering yields,that is, the rate of sputtering or yield of tantalum decreases withincreasing angle of incidence of the impinging ion. Since the angle ofincidence of ions striking vanes 12 is relatively small, high yields areexperienced. on the other hand, those ions which fail to strike vanes 12and impact on the top or bottom surface of the fourth electrodes '5, '5'do so at a nearly normal angle of incidence. Consequently, the negativepotential on auxiliary cathode 5 can be increased to increase pumpingbeyond that which would be possible if the sputter cathodes 6, 6' andfourth electrodes 5, 5' were made of a material which did not exhibitthe angular dependence of sputter yields seen with tantalum.

It will now be obvious to those skilled in the art that otherembodiments and arrangements are within the scope of the invention.Various anode geometries may be used, for example, rectangular insteadof cylindrical. Further, the pump assembly 3 may 'be inserted directlyin the chamber to be evacuated without the need for a pump housing 1,provided a grounded plate is provided adjacent outwardly of the sputtercathodes 6, 6 to fix the potential.

Accordingly, the description and accompanying drawing are to beconsidered as illustrative only and shall not be construed asrestricting the scope of the invention as hereinafter defined.

What is claimed is:

1. An ion pump apparatus comprising: an apertured anode member; anapertured electrode insulatingly disposed adjacent one end of said anodemember; and an apertured sputter cathode insulatin-gly disposed adjacentto and outwardly of said apertured electrode and adapted to bemaintained at a sufliciently high negative potential to insuresputtering of material therefrom.

2. An ion pump apparatus according to claim 1 Wherein the diameter ofthe aperture in said apertured electrode is approximately of thediameter of the aperture in said apertured anode member.

3. An ion pump apparatus according to claim 1 wherein said aperturedelectrode is adapted to be maintained at a negative potentialintermediate the potential applied to said apertured anode member andsaid apertured sputter cathode.

4. An ion pump apparatus according to claim 1 comprising means formaintaining said apertured anode member at ground potential, means formaintaining said apertured sputter cathode at a high negative potential;and means for maintaining said apertured electrode at a potentialintermediate the potential applied to said apertured anode member andsaid apertured sputter cathode.

5. An ion pump apparatus according to claim 3 wherein said aperturedsputter cathode comprises one or more elongated members in a planenormal to the axis of aperture of said apertured anode member.

'6. An ion pump apparatus according to claim 5 wherein said elongatedmembers comprise one or more elongated thin rectangular metal strips.

7. An ion pump apparatus according to claim 6 wherein said metal stripsare made of tantalum.

8. An ion pump apparatus according to claim 6 wherein said metal stripsand said apertured electrode are made of tantalum.

References Cited UNITED STATES PATENTS 3,292,844 12/1966 MacKenZie230-69 FOREIGN PATENTS 1,289,986 2/1962 France.

ROBERT M. WALKER, Primary Examiner.

US. Cl. X.R. 313-7

